Plastic collapse assessment procedure for vessels with deep local thin area subjected to internal pressure

There are numerous cases in which the external surfaces of vessels such as towers, piping and storage tanks are partially corroded under their insulation. Some considerations on the plastic condition of cylinders with a local thin area have been reported, but a simplified evaluation procedure has not been established for the case in which a cylinder is simultaneously subjected to internal pressure and external bending moment due to earthquake, etc. Recently, the Ibaraki FFS rule has been presented, but the limitation on flaw depth (a) in the Ibaraki FFS rule is less than 0.7 (=a/t) of the original wall thickness (t). The effective cross section for the reference stress solution used to predict the plastic collapsed loads was investigated by nonlinear FEA. For the case in which a flaw depth to thickness ratio is large (a/t ≦ 0.7–0.95), the reference stress solution which decreases the effective cross section as a function of the flaw depth ratio was proposed.